According to the inductor current condition, switching regulators can be generally categorized into two types: Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM). FIGS. 1 and 2 are waveform diagrams of the ideal inductor currents in CCM and DCM switching regulators, respectively, and both the modes have respective advantages and applications. FIG. 3 shows a conventional DCM synchronous buck switching regulator 100, in which a controller 101 switches a high-side switch 110 and a low-side switch 112 connected in series between a power input terminal Vin and a ground terminal GND to generate an inductor current IL that charges a capacitor Co to produce an output voltage Vout, and voltage divider resistors R1 and R2 are connected in series between the output terminal Vout and the ground terminal GND to generate a feedback voltage VFB for the controller 101 to regulate the output voltage Vout. In the controller 101, an error amplifier and compensator unit 102 generates an error signal VEA according to the feedback voltage VFB, a Pulse Width Modulation (PWM) comparator 104 generates a PWM signal according to the error signal VEA, a logic and driver unit 106 switches the switches 110 and 112 in response to the PWM signal to convert the input voltage Vin to the output voltage Vout, and a zero-current sense circuit 108 senses the inductor current IL by sensing the voltage on the phase node LX, and triggers a zero-current detection signal ZCDET for the logic and driver unit 106 to turn off the switch 112 when the inductor current IL falls down to a zero-current threshold Ith during the switch 112 is on, in order to prevent reverse inductor current IL from the output terminal Vout.
FIG. 4 is a waveform diagram showing the inductor current IL and the voltage on the phase node LX of the switching regulator 100, in which waveform 200 represents the inductor current IL, and waveform 202 represents the voltage on the phase node LX. At time t1, the high-side switch 110 is turned on and the low-side switch 112 is turned off, and thereby the inductor current IL begins to increase with the slopeslope_rise=(Vin−Vout)/L.  [Eq-1]At time t2, the high-side switch 110 is turned off and the low-side switch 112 is turned on, and thereby the inductor current IL begins to decrease with the slopeslope_fall=Vout/L.  [Eq-2]When the inductor current IL falls down to the zero-current threshold Ith, as shown at time t3, the low-side switch 112 is turned off to prevent reverse inductor current IL from the output terminal Vout. As shown by the waveform 202 in FIG. 4, between times t3 and t4, a body diode 114 of the switch 112 is turned on to remain the inductor current IL flowing from the ground terminal GND to the output terminal Vout through the phase node LX, and until the inductor current IL becomes zero the body diode 114 turns off.
However, the zero-current threshold Ith of the conventional zero-current sense circuit 108 is fixed but not adjustable when it is designed, and in the event that the output voltage Vout or the inductor L is changed, causing the falling slope slope_fall of the inductor current IL changed, the pre-set zero-current threshold Ith becomes not suitable to the real conditions. For example, when the falling slope slope_fall of the inductor current IL is changed to be much steeper as shown by the waveform 204 in FIG. 5, the low-side switch 112 may not be turned off in time after the inductor current IL falls down to the zero-current threshold Ith, the voltage on the phase node LX may become positive eventually, as shown by the waveform 206 in FIG. 5, and a reverse inductor current IL occurs. On the contrary, when the falling slope slope_fall of the inductor current IL is changed to be much gentler as shown by the waveform 208 in FIG. 6, the body diode 114 is eventually conductive for a longer time period after the inductor current IL falls down to the zero-current threshold Ith, as shown by the waveform 210 in FIG. 6, and more wasted power consumption occurs.
Therefore, it is desired a zero-current sense apparatus and method with an adjustable zero-current threshold for a switching regulator.